Etale Homotopy

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100 M. Artin · B. Mazur

Etale Homotopy

Springer-Verlag Berlin Heidelberg New York Tokyo

Lecture Notes in Mathematics Edited by A. Dold and B. Eckmann

100 M. Artin · B. Mazur

Etale Homotopy

Springer-Verlag Berlin Heidelberg New York Tokyo

Authors Michael Artin Massachusetts Institute of Technology Rm 2-239, Cambridge, MA 02139, USA Barry Mazur Harvard University One Oxford St., Cambridge, MA 02138, USA

1st Edition 1969 2nd Printing 1986

ISBN 3-540-04619-4 Springer-Verlag Berlin Heidelberg New York Tokyo ISBN 0-387-04619-4 Springer-Verlag New York Heidelberg Berlin Tokyo

This work is subject to copyright. All rights are reserved. whether the whole or part of the material is concerned, specifically those of translation. reprinting. re-use of illustrations. broadcasting. reproduction by photocopying machine or similar means. and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to 'Verwertungsgesellschaft Wort". Munich.

© by Springer-Verlag Berlin Heidelberg 1969 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2146/3140-543210

Contents

................. ...................

1.

A glossary of the categories in which we shall work, and fibre resolutions.

2.

Pro-objects in the homotopy category••••••••••••••••••••••• 20

3.

Completions ••

4.

Cohomological criteria for

5.

Completions and fib rations.

6.

Homotopy groups of completions ••••••••••••••••••••••••••••• 70

7.

Stable results.

6

....... .... ................... ..... ..... ..... 25

g -isomorphism••••••••••••••••• 35 ......... .. .... ....... ....... .. 60

8.

........................................... 75 Hypercove rings. ........................................... 93 The Verdier functor •••• ............................... ••••• 111

10.

The fundamental group••••••••••.••••••••••••••••••••••••••• 117

11.

A profiniteness theorem•••••••••••••••••••••••••••••••••••• 124

12.

Comparison theorems •••••••••••••••••••••••••••••••••••••••• 129

Appendix: 1.

Lim1 ts •.................................................... 147

2.

Pro-objects and pro-representable functors.

3.

Morphisms

4.

Exactness

............... .154 of pro-objects ••••••••••••••••• . ...... ..........•159 properties of the pro-category. ................. .163

References .................................•.....•.........•... 167

- 1 -

These notes are an expansion of the results announced in [2].

The material was presented in a seminar at Harvard

University during the academic year 165- 166.* Our aim is to study the analogues of homotopy invariants which can be obtained from varieties by using the etale topology of Grothendieck.

Using the constructions of Lubkin [21]

or Verdier [3], we associate to any locally noetherian prescheme

X a pro-object in the homotopy category of simplicial

sets (cf. 2), which we call the etale homotopy prescheme

X.

of the

For a normal variety over the field of complex

numbers, we show that

Xe t is a certain profinite c